Oxygen isotopic compositions of the late orogenic granites in the Southern Piedmont of the Appalachian Mountains,U.S.A., and their relationship to subcrustal structures and lithologies

Data were acquired from 143 whole rock samples from 20 late orogenic, post-metamorphic, Hercynian-age granitic plutons from the Piedmont of the southern Appalachians, principally in Georgia and South Carolina. These plutons exhibit a regional gradient in oxygen isotopic compositions in which the granites confined to the Inner Piedmont to the northwest are¹⁸O-enriched (11.4 to 7.9) whereas those toward the southeast within the Charlotte-Slate and portions of the Kiokee belts have distinctly lower¹⁸O/¹⁶O compositions (8.2 to 5.5); one body that lies along the southeastern edge of the Piedmont in South Carolina, however, appears to be anomalously¹⁸O-enriched (8.9). Most plutons display¹⁸O/¹⁶O variations of <1‰ although two vary by as much as 3–4‰. The regional oxygen isotopic pattern among plutons appears to correlate directly with: (1) regional Bouguer gravity patterns, in which the¹⁸O-enriched plutons occupy areas characterized by negative anomalies, whereas low-¹⁸O bodies are invariably restricted to regions of positive anomalies; (2)⁸⁷Sr/⁸⁶Sr data, where granites with δ¹⁸O values <8‰ have low initial strontium ratios of ～0.703 to 0.705, while¹⁸O-enriched plutons (>9‰) have ratios >0.710; (3) contrasting chemical and accessory mineral compositions, in which many¹⁸O-depleted granites have a number of I-type characteristics, whereas several of the most¹⁸O-enriched plutons exhibit a number of S-type features. It can be inferred from these data that the¹⁸O-enriched granites were formed from continental crustal protoliths that underlie much of the Inner Piedmont and portions of the Kiokee belt, whereas the low-¹⁸O plutons were derived from more mafic sources beneath the Charlotte-Slate and portions of the Kiokee belt. The overall correspondence between the regional¹⁸O/¹⁶O patterns exhibited by the granites and gravity data implies that these grantes may be essentially rooted to their protoliths, in turn suggesting that the large-scale translational movement recently proposed for the Southern Piedmont may have occurred prior to intrusion of these granites ～320 m.y. ago.